[NEXUS-5831] Create artifacts/content jars along artifacts/content xmls

Signed-off-by: Alin Dreghiciu <adreghiciu@gmail.com>
[NEXUS-5831] Fix assertions about existing files in P2 repos

Signed-off-by: Alin Dreghiciu <adreghiciu@gmail.com>
[NEXUS-5831] Use Java7 try-with-resource

Signed-off-by: Alin Dreghiciu <adreghiciu@gmail.com>
Merge pull request #130 from sonatype/NEXUS-5831-p2-jarred-metadata

REVIEW: [NEXUS-5831] Create artifacts/content jars along artifacts/content xmls